1. Field of the Invention
The present invention relates to a heating device.
2. Description of Related Art
In semiconductor device manufacturing processes using semiconductor manufacturing apparatuses, heating is performed for the purposes of forming an oxidized film on a wafer and the like. One of heating devices used to heat wafers includes: a disk-shaped ceramic base having a heating surface on which a wafer as a heating object is placed and heated; and a resistance heating element embedded in the ceramic base. This heating device is advantageously suitable for not only a film formation apparatus used in a semiconductor manufacturing process but also a surface processing apparatus which performs dry etching for the surface of a plate-shaped material and the like.
In some cases, the wafer undergoes film formation by plasma CVD, plasma etching, or the like during heating by the heating device. A heating device which is capable of forming a plasma atmosphere around the wafer includes a plate-shaped high frequency electrode embedded near the heating surface in the ceramic base substantially in parallel to the heating surface. In the rear surface of the ceramic base opposite to the heating surface, a hole into which a terminal introducing power to the high frequency electrode is inserted is formed toward the high frequency electrode. In the bottom surface of the hole, the high frequency electrode itself or a metallic conductive member connected to the high frequency electrode is exposed and joined by brazing to the terminal inserted in the hole so as to be electrically continuous to the terminal. This terminal is connected to a power supply for supply of power to the high frequency electrode.
In one of the heating devices each including such a high frequency electrode (Japanese Patent Laid-open Publication No. 8-277173), a mesh high frequency electrode is embedded in the ceramic base of aluminum nitride. The high frequency electrode is exposed to the hole of the ceramic base and joined to a Ni terminal with a brazing material.
In another one of the heating devices each including such a high frequency electrode, koval, whose thermal expansion coefficient is intermediate between Mo and Ni, is interposed between a Mo mesh high frequency electrode and a Ni terminal. Mo, koval, and Ni are joined to one another by a brazing material. In still another type thereof (Japanese Patent, Laid-open Publication No. 2002-134590, Japanese Patent Specification No. 3790000), instead of directly joining the Mo mesh high frequency electrode and the koval material, the Mo mesh high frequency electrode is co-sintered with a Mo bulk material having a diameter of 3 mm and a thickness of 2 mm as a conductive member, and the Mo bulk material is joined to the koval material.
Each of such high frequency electrodes is configured to have a planar shape parallel to the heating surface of the ceramic base in order to equalize distribution of plasma generated on the heating object. A part between the high frequency electrode and the heating surface of the ceramic base serves as a dielectric layer or an insulating layer, for example, made of aluminum nitride, which is a material of the ceramic base, and has a thickness of about 1 mm for equalization of the plasma distribution.
Such a structure of the high frequency electrodes is employed in heaters, electrostatic chucks, and susceptors in common.
A general aluminum nitride ceramic base with a high frequency electrode mesh embedded therein has a thickness of about, 5 to 25 mm. In this thickness of the ceramic base, thickness of an aluminum nitride layer serving as the dielectric layer or insulating layer between the high frequency electrode and heating surface is about 1 mm as described above. The ceramic base has a part where the hole is formed, into which the terminal connected to the high frequency electrode is inserted. The part between the bottom of the hole and heating surface is structurally the thinnest and weakest in the ceramic base. Accordingly, cracks might occur in this part.
For example, in a part of the region between the high frequency electrode and the heating surface which faces the hole into which the terminal is inserted, push force is applied when an external conductive connector is assembled and connected to the terminal, and cracks may occur. Moreover, a part of the heating surface of the ceramic base around the top of the terminal becomes colder than the other part during heating of the object because of heat transfer to the terminal. This might produce thermal stress and cause cracks near the terminal in the heating surface of the ceramic base. In the heating device including a Mo bulk material between the hole into which the terminal is inserted and the high frequency electrode, cracks might occur near the terminal because of the difference in thermal expansion coefficient between the Mo bulk material and the aluminum nitride base. These cracks were highly likely to be induced by corrosion of the heating surface due to long-term use of the heating device.
An object of the present invention is therefore to provide a heating device with high reliability and long life by advantageously preventing cracks from occurring near the terminal in the high frequency electrode.